usb/hcd-ehci-sysbus: Convert to QOM realize

usb/hcd-ehci-sysbus: Convert to QOM realize

The SysBus qdev initfn merely calls SysBusDeviceClass::init, so we can
replace it with a realizefn already. This avoids getting into any initfn
ambiguity with the upcoming Faraday EHCI implementation.

Rename internal usb_ehci_initfn() to usb_ehci_realize() to allow to
return Errors from common initialization code as well.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>